When something is sent through the mail, it must be placed in a package, given the proper address, and moved through the system at the right time. It is no different when a packet of information is sent through a network. The protocol for how a packet is sent through a network may be defined by a network standard. The dominant standards for wireless local area networks (WLANs) are those of the IEEE 802.11 family. The 802.11 family of standards specify how information sent through the network should be packaged and how it should be addressed. A single 802.11 specification may define the protocol for both a physical layer (PHY) and media access control layer (MAC) of the communication transmission scheme. These two layers combine to prepare information for successful transmission through a wireless network.
The PHY is the first layer in the seven-layer open system interconnection (OSI) model for layered communications. The PHY defines the manner in which the raw zeros and ones that comprise a signal will be grouped into code words or symbols and then converted into a physical signal. The PHY also defines how this physical signal is transmitted through a physical link such as a cable or the air. In a wireless network, some information regarding the transmitted signal, such as modulation scheme and/or number of streams may be sent to one or more receiving devices. In a wireless communication device, the steps necessary to implement the PHY layer packaging may be accomplished by a radio frequency processing radio component, and a base band processing component. The radio modulates the signal in accordance with the relevant standard. Example modulation techniques include quadrature amplitude modulation (QAM), amplitude modulation (AM), and frequency modulation (FM).
The MAC layer is a sub-layer of the second layer of the OSI model. The MAC layer provides addressing and channel access control mechanisms so that several stations can communicate within a network. Medium access must be controlled because if different stations within a network broadcast at-will, the air would be filled with conflicting signals. Just as a group of people sitting in a room need to learn to take turns talking so that everyone can be heard, collision avoidance systems are necessary so that multiple stations on a wireless network are not talking at the same time thereby destroying the transmitted information. The channel access system used in the family of 802.11 standards is called carrier sense multiple access with collision avoidance mechanism (CSMA/CA). The operation of CSMA/CA can be described with reference to FIG. 1.
FIG. 1 illustrates four stations in a wireless network; station 100, station 101, station 102, and station 103. Whenever one of the stations wishes to access the medium it will sense the channel, basically listening to hear if someone else is talking. If the channel is busy, the station will wait a random amount of time, and then try to transmit again. Range indicator 104 shows the area over which station 100 can be heard. Range indicator 105 likewise shows the area over which station 101 can be heard. The configuration of stations and range indicators in FIG. 1 illustrates an initial problem with carrier sense collision avoidance methods. Since station 103 is outside range indicator 104, if station 100 is transmitting a message to station 101, station 103 will be unable to sense the transmission. Therefore, without an additional system, station 103 will begin transmitting, and station 101 may be overloaded and unable to understand the message from station 100.
The collision avoidance scheme used by the 802.11 family of standards may involve the transmission of two signals called request to send (RTS) and clear to send (CTS). The process can best be explained with reference again to FIG. 1. When station 100 wants to communicate with station 101, it will first check the medium to make sure it is clear, and will then send out an RTS signal. The RTS signal will be received by any stations within range indicator 104, which includes both station 102 and station 101. All stations on the network will have internal network allocation vectors (NAVs) that they can set in response to information contained within the RTS signal. As long as a station's NAV is not zero, it will continue to wait and act as if the channel is taken. In addition, when station 101 receives an RTS signal that indicates it will be the recipient of a packet, it will send out a CTS signal to all stations within range indicator 105, which includes station 103. Station 103 will in turn set its NAV with information contained within the CTS signal. In this way, collision avoidance is provided for the transmission from station 100 to station 101. The information within the CTS and RTS signals that are used to set the stations' NAVs that is necessary to orchestrate this system is delivered through headers that are added on to the message packets as they travel down through the OSI layers.
In the OSI model, each layer provides services to the layer above and receives services from the layer below. Headers are added on to data as it moves down towards transmission through the physical medium such that the data is continually encapsulated and repackaged into a format that the recipient layer can operate on. With reference to FIG. 2, packet 200 is comprised of PHY header 201, MAC header 202, miscellaneous headers 203, and payload 204. PHY header 201 is added onto the signal as it moves from the MAC layer into the PHY layer. Likewise, miscellaneous headers 203 and MAC header 202 are added onto the signal as it moves into the corresponding OSI layer. Headers are also commonly referred to as preambles.
A critical portion of PHY header 201 is the physical layer convergence protocol (PLOP). The PLOP contains, among other items, a bit stream that represents the length of the packet, and the rate at which the packet is being transmitted in bits per second. The PHY uses this information to properly detect the end of the packet, as this information will indicate the time it will take for the signal to be sent through the physical medium. This is extremely important for WLAN that are deployed indoors where there are several different paths a signal can take as it bounces around inside a building. In such a multipath environment, knowing the length of the signal is one way in which the PHY can screen out the effects of these reflections.
MAC header 202 comprises, among other items, a frame control field which tells the recipient station what kind of data to expect, a duration field, and address fields. The address fields include a sender address, a recipient address, and an access point address. The address fields aid the network in determining where the packet is going, and how to route a packet through the network. The duration field is the field that is used by the MAC to set the NAV values for stations that receive the packet and header. Before sending a CTS or an RTS signal out, a station will determine how long the packet transaction will take, and will program the duration field based on this calculation. It is through this process that the MAC header information sets when an individual station can access the medium.
In order for CSMA/CA to work, stations from different iterations of the 802.11 family need to be able to communicate to prior iterations. If a legacy station is unable to hear messages broadcasted by a modern station, the whole system of carrier sensing will fail. With reference to FIG. 1, assume that station 100 and station 101 are modern stations and station 102 is a legacy station. In a system that did not account for backward compatibility, when station 100 began broadcasting for purposes of sending a message to 101, station 102 would be unable to hear this broadcast. Therefore, when station 101 sends an acknowledgment signal back to station 100 to indicate that the information was received, it is possible that station 102 may have begun broadcasting and fatally interfere with the exchange between station 100 and station 101. If legacy stations broadcast independently of modern stations the medium will be taken up by chatter and the network will fail.
To allow for backwards compatibility, modern stations such as those configured to operate under the 802.11n and draft 802.11ac standards can be configured to produce packets that contain a unique form of PHY header. These PHY headers are comprised of two parts; a legacy physical layer (PHY) preamble comprising a legacy rate field and a legacy PHY length field, and a separate physical layer preamble that comprises the PLOP of the modern standard. The legacy PHY preamble is also sometimes referred to as the legacy spoof preamble. The legacy PHY preamble is always transmitted using six megabits per second (Mbps). At this rate, any 802.11 device using orthogonal-frequency division multiplexing (OFDM) will be able to decode the legacy PHY preamble. However, the legacy stations will not be able to decode the separate physical layer preamble, or any of the remaining portions of the packet. As a result, the legacy stations will keep quiet and be ultimately unaffected by the modern messages. This system will result in a quiet channel regardless of modern packet transfer in a network comprising legacy systems. This is important because oftentimes a user or network administrator cannot control the devices that are within the area of influence of a WLAN. It is therefore not suitable to allow the introduction of a legacy system to cause total failure of the network.
Payload 204 is the actual information that a user desires to transmit. Although PHY header 201 and its subsidiary legacy PHY preamble, MAC header 202, and miscellaneous headers 203 are absolutely necessary for payload 204 to be transmitted through the network, they are wasted space from the perspective of a perfectly efficient system. Although one cannot send a letter through the mail without an envelope, the weight of the envelope does affect the shipping cost of the letter. Given that modern wireless networks are able to send hundreds of thousands of packets per second, any minor decrease in the number of bits, or improvement in the information content in a packet header could lead to dramatic improvements in a network's overall efficiency and performance.